Catalase is one of the critical antioxident enzymes that constitutes a major defense system against superoxide and hydrogen peroxide by converting H2O2 to H2O and O2. The bovine catalase structure complexed with NADPH was solved to 2.5 E a decade ago. Since that time, the role of the bound NADPH is poorly understood and the structure of the human enzyme, despite its critical role in regulation of the cellular redox potential in apoptosis, response to pathogens, DNA damage and aging, has been ignored. Given the central role of catalase in humans, we wish to examine the structural basis of mRNA binding by human catalase through crystallography.